It is known that valuable documents, e.g. bank-notes, identity cards, pass-ports etc. can be provided with invisible marks which can be made visible with the help of external means or visible marks which change in visual character under some externally applied condition. It is known, for example, that on identity documents a marking of legitimacy or of authenticity may be fixed which is of phosphorescent "colour" or material which can be made visible through illumination with UV-light. In such a manner any type of text and markings such as numerical series, names etc. can be introduced. It is an inconvenience to check of marking of legitimacy in that it requires special auxiliary means in order to make the marking visible, and to carry out a check of legitimacy it is necessary therefore, to have access to, or to call on, such equipment. It is also known that parts of a document can be magnetized and the documents be provided with a code of legitimacy produced by magnetic means. In this case too, access to a relatively complicated apparatus is required in order to establish the presence and significance of the magnetic markings.
Thus there is clear need for a simpler mark of legitimacy or authenticity which may be detected without the use of special machinery.